RACK1 and Menkes disease: Of the four proteins that were undetected in the MD eyes as compared to the normal control eyes, and thus heavily down-regulated during formation of MD, adenine phosphoribosyltransferase (APRT) is an intracellular enzyme that contributes to formation of AMP by catalyzing the phosphoribosylation of adenine, peroxiredoxin 1 acts to protect cells from oxidation, βB2-crystallin is involved in post-translational modification of proteins, and Gnb2l1 (guanine nucleotide binding protein [G protein], beta polypeptide 2-like 1) mediates signal transduction.